Free machining steel



Patented Oct. 18, 1949 UNITED STATES; PATENT oF-FrcE FREE MACHINING. STEEL Samuel L. Gase, Columbus, Ohio; assignor, by

mesne assignments, to Carnegie-Illinois Steel: Gorporation, Pittsburgh, 2a., a corporation. of New-Jersey No'Drawing. Application January 29",,194'7, Serial\No.-725,170

2 Claims. (Cl..75.-123) This-inventionrelates to steels that are freely Eje t. iili machinable and; more particularlyto free machining steels that are readily forgeable and :which Mama-mi havergoodductility together: withan-improved SWQINOI 1' M11 P S Si tr: method of making. the same. Index Heretofora. sulphur has: been addedto steel' to Per cent Per cent Per cent Per cent Per cent .12 .96 .106 .003

improve thezmachinability thereon This is quite 4 138 gm efiective-butthe results vary widelyyfromheat .to 3 4 111.4 heat. Momever, in: order for the sulphur tube 09 74 104 I132 003 i efiective, considerable amounts thereotwerenec- 10 9 12 33 {38 3 25 essary which tends to make the steel hot-short. 11 82 102 1 044 6 and'ithereforesnoti readily forgeable. To offset this tendency, the manganese content had to be have discovered t t; in combination t increased and th sin turniended' to lower the theabove specified'low silicon content, it" is demachinability and to render the steel too hard sirable toincrease-the.phosphorus content for ce p po es. Thus, it. is desirable to point somewhat higher than is customary for obt machinability with. n m m sulphur steels 'of'this type: The increase in machinability additions. Acco d y; it 'J of my to be gained by increasedphosphorusis clearly invention to provide free machining steels of Shown in the followingitablez uniform quality and at the-same:time thathave enhancedmachining qualities; without. sacrifice Mahm offorgeability or ductility. It is-afurtherobject SteelNo. 0 M11 P s 1 Si ability" to provideta method for commercially making I Index. the steels of thisinvention.

In my improved'free machining steel, the cargg 8- o 8g bonshould" be; maintained' below;.08%',.as'- carbon i 01 64 0 200 01108 0 005 112 has: a..deleterious' effect ommachinability, as is 1 0 115 shown by. thefollowing. chart:

From this it is seen that increasing the per- Machma centage of phosphorus in a sulphur-bearing low- S Si g y silicon steel improves the machinability thereof. To obtain the benefits of phosphorus the content 0 126 o 002 117 5 thereof should be at least .06% and preferably at 01137 01002 90: 5 least .08%. Due to certain unfavorable reactions, 0 13 2 5 it is desirable to limit the phosphorus to a maximum of 25% even though larger amounts will I have determined that very beneficial results have a beneficial effect on machinability.

so far as machinability is concerned, can be In order to obtain optimum beneficial results obtained by maintaining the silicon content of from sulphur additions, it has heretofore been free machining steels below .01%. This should necessary to add at least .15% sulphur. By mainresult in an oxygen content of .04% when the taining the silicon and manganese low and the carbon is at the maximum of .08% and correphosphorus high, I have determined as shown spondingly higher oxygen contents for lower in the following table that good machinability carbon contents. The greatly improved machincan be obtained with considerably less sulphur,

ability resulting from the silicon content being i.e., as low as .05% which is considerably less below .01% is clearly shown in the following than was heretofore considered necessary. Howtable. ever, if even more enhanced machinability is desired, the sulphur can be increased to .30% and the manganese correspondingly increased to 1.00%. A minimum of .45% manganese should be used with the minimum sulphur content. The effect of varying manganese and sulphur content is shown in the following table:

-While the exact explanation of the mechanism leading to the improved machinability of highsulphur steels is not definitely known at present, a hypothesis may be offered that the higher oxygen content introduces the changes in the FeMnS--O system which affects the. nature of the manganese-sulphur compounds embedded in the steel matrix which in turn influences the cutting properties. Evidence available is not conclusive regarding the exact composition of these compounds, but it is definitely established that low-silicon high-oxygen steels of the type herein disclosed having good machining properties have heavy inclusions of these types of a generally globular shape. In steels of average machinability, these inclusions incline to be of an elongated shape which suggests that at rolling temperatures, they are less hard and more ductile than those in the low-silicon high-oxygen steels of this invention.

There is at present no method of obtaining commercially a free-cutting Bessemer steel having a maximum silicon content of .0l%. The equilibrium constant for residual silicon and FeO is strongly influenced by the temperature thereof. For instance, .05% FeO is in equilibrium With .02% silicon at 2770 F. and with 35 times as much silicon (.'70%) at 3100 F. This apparently is one of the fundamental reasons for the wide variability in machining qualities of Bessemer screw grade steels. Therefore, it is essential in obtaining uniformly low silicon in such stee1 to blow it as cold as possible.

In acid Bessemer blowing practice, the preponderant source of heat is the oxidation of silicon during the first part of the blow. Thus an iron charge having a uniformly low silicon content is a prime requisite of obtaining the low silicon contents of the steel of my invention. This may be accomplished by adding a suflicient amount of dried roll scale or iron ore to the molten iron before charging it to the converter. Experiments have shown that such additions can reduce the time of the silicon blow by 3 or 4 minutes. The iron ore or scale should be added to the charge by placing it on the bottom of the ladle and pouring the molten iron from the mixer on top thereof. This causes a violent agitation and instant reaction with the formation of a heavy iron-manganese-silicate slag which should be skimmed off when charging the low-silicon iron into the converter. The cm or scale should not be successfully added to the converter because it causes a thin watery slag and frothing and splitting during the blow. Moreover, greatly increased quantities of ore or scale are required.

While I have shown and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that variou other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. A free cutting Bessemer steel containing not more than .08% carbon, .05 to .30% sulphur, .45 to 1.00% manganese, .06 to .25% phosphorus and less than .01% silicon, the balance iron and incidental impurities.

2. A free cutting Bessemer steel containing not more than 08% carbon, .05 to .30% sulphur, .45

to 1.00% manganese, the manganese content being at the lower end of the manganese range when the sulphur content is at the lower end of the sulphur range, .06 to .25% phosphorus and less than .01% silicon, the balance iron and incidental impurities.

SAMUEL L. CASE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of 1937, American Institute of Mining and Metallurgical Engineers, Iron and Steel Division, vol. 125, pages 248 and 274. 

